high humidity, is it bad?

[abumuhannadh]

heat + high humidity is very uncomfortable

but i am not here to talk about that

my question is,

what are the effects of high humidity on seeing?

is it good or bad?

and by high humidity i mean 75% and above.

thanks

-Abu Muhannadh-

[FraserClarke]

Interesting topic. Probably depends a lot on your location. A lot of the american sites report very good seeing on very humid (>90%) nights in the south/east of the US. I would have thought, possibly naively, that high humidity is correlated with low ground-level winds, which could lead to more stable conditions? Most seeing at good sites though is dominated by the jet stream layer at about 8-10km, so presumably not related to humidity.

I would also think that high humidity gives you worse transparency (more water vapour between you and the sky!). So, humid nights might be great for planets and double stars, but bad for faint galaxies.

One thing to watch out for though is that high humidity can be a risk to your optics; because you're more likely to get dew forming on them. Just need to pay attention to it a bit more.

[abumuhannadh]

humid nights might be great for planets and double stars, but bad for faint galaxies.

why does that happen? doesn't the seeing condition work the same on both planets, stars and galaxies? (sorry i am not very good at science )

One thing to watch out for though is that high humidity can be a risk to your optics; because you're more likely to get dew forming on them. Just need to pay attention to it a bit more.

oh, i don't have a scope, yet. but gonna have a refractor soon. i guess dewing is not a problem with refractors?

and i don't think dewing actually happens at my place (the temperature doesn't vary much)

[acey]

Dewing happens if optics get cool enough to fall below the dew-point. I don't know how the figures work out, but whenever I've observed in warm humid places (not Britain!) I've never had dewing, I've only encountered it in cold conditions. If cars parked in the open-air overnight get dew on their roof then a scope will dew too. If cars don't dew then I wouldn't expect a scope to.

Misty conditions are associated with still air and that's good for planets, where you want the steadiest possible view and aren't concerned about light loss due to absorption by water vapour. But for galaxies you want the absolute minimum light loss and aren't so concerned about air stability (since the eye can't resolve small details at low luminosity). For viewing galaxies, water vapour in the atmosphere is undesirable because not only does it absorb the light from galaxies, but it also reflects and scatters terrestrial light pollution. Planets aren't hurt by light pollution - they're bright.

So astronomers tend to use "seeing" to mean air stability and "transparency" to mean sky darkness (lack of absorption). Planetary observers want good seeing, galaxy observers want good transparency.

Any type of scope will suffer from dewing if the conditions allow for it to form. First line of defence is a dewshield and if that doesn't work there are hair-dryers, dew strips etc. But if the temperature can't fall low enough for dew to form then it's not an issue.

For galaxies the most important thing is the darkness of the sky, so light pollution is the number one concern, ahead of any other factor. If there is high humidity but no condensation (i.e. the air is warm) then I shouldn't have thought it would lead to any significant loss of transparency.

[brianb]

75% RH? I'd call that DRY ..... mind you I call 20C HOT and 25C UNBEARABLE so perhaps I'm not the best judge. However fighting dew goes with maritime (coastal) sites & you don't get dew with RH below about 90%.

If you have a good site, excellent seeing goes with still air which is often slightly misty. If you have a poor site, still air just makes the heat plumes from local boilers etc. more of a nuisance, a light breeze stirs up the air & helps prevent strong turbulence from occurring ... but you still won't get the sort of rock solid image that the better sites sometimes are blessed with.

[FraserClarke]

why does that happen? doesn't the seeing condition work the same on both planets, stars and galaxies? (sorry i am not very good at science )

Wot acey said

[abumuhannadh]

thanks guys,

i was talking about Maldives (thats my home). humidity never falls below 75% there (at least not i heard of) and the temperature is between 26C and 33C all year round. seldom falls below 26C but not below 23C. some people speaks about seeing morning dew on leaves, but i have never seen, so i am not sure.

light pollution is not a concern in my small town, the sky is pretty dark after 7:30pm to 5:00 am everyday (Maldives is on the equator, so daylight time stays the same all year round)

here is a 30sec exposure of Scorpios i took about two years ago, from inside my house.

as i am gonna have a small aperture refractor, my main targets will be the moon and the big planets. so i am having the right condition then, i am glad.

-Abu Muhannadh-

[acey]

Lovely! With those conditions you should find a small refractor is great for deep-sky too - check out the Milky Way with it.

[abumuhannadh]

75% RH? I'd call that DRY ..... mind you I call 20C HOT and 25C UNBEARABLE so perhaps I'm not the best judge.

wow:eek:, i call 20C cold and 25C cool.

if 25 is unbearable for you, i wonder what would it be like for you to feel the heat where i am now (Saudi Arabia). these days afternoon temperature averages 46C

but it is very dry, (4% to 11% humidity) so i do not complain. i can bear heat, (without humidity that is) cold is what i struggle with.

[abumuhannadh]

Lovely! With those conditions you should find a small refractor is great for deep-sky too - check out the Milky Way with it.

thanks, thats encouraging

actually i can make out the milky way naked eye when i am there.

[FraserClarke]

here is a 30sec exposure of Scorpios i took about two years ago, from inside my house.

as i am gonna have a small aperture refractor, my main targets will be the moon and the big planets. so i am having the right condition then, i am glad.

-Abu Muhannadh-

Fantastic. I'd take 95% humidity and an equatorial dark sky location any time

A few messier objects in your 30s image there. M6, M7, M8, M22 -- Definitely spend some time scanning the milky-way with your refractor too

[Tarrabas]

Hello

This is my first post here. To introduce myself. I´m a student of chemistry in middle Europe (Austria. Not to be mistaken for Australia. There are no deadly penguins, poisonous spiders and only few deadly poisonous snakes in Austria.). I must admit i´m a naked eye night sky observer (I own no Telescope.)

Now that the introduction is over i have an additional question regarding air humidity. The night sky above my town seems extreme bright the last days. It could have something to do with the eruption of the Eyjafjallajökull in Iceland, which has disrupted Air travel in Europe for several days. The late evening Sky after the sunset had a strange greenish to turqoise tone, (from about 21:00 pm to 22:00 pm) due to the volcanic ash in higher air cavities. Now midlle europe has a quite tight settling and light pollution is a real problem when watching the night sky, except when you go up to the Alps. The town i live in has about 200.000 inhabitants and there are the small villiges from 10 to 10.000 inhabitants near it. Now my question is. Could the high humidity in the upper cavyties cause the brighter night sky? Or are my observations wrong and the night sky isn´t brighter as normal? (All observations are subject to individual error.) Sorry because of bad english. I´m no native speaker.

Tarrabas

[yeti monster]

I don't know the meteorologicl facts invovled, but my experience is like this:

Wednesday and Thursday nights this week have been very hot and humid here, I've been sitting in just shorts, sweat dripping off me and getting into my eyes, and they've been the clearest nights I've ever had for observing, being able to make out DSOs with the nekkid eye, something which I hadn't experienced before.

I store the wee skymax 90 in the BOP HPU control room, as it isn't air conditioned so no condensation, but when I returned here on Tuesday it had been stashed in the Subsea workshop which is air conditioned, for 4 weeks, the darn thing took forever to acclimatise!

[brianb]

Welcome to the forum!

I think the Icleandic volcano with the unspellable name has long since ceased erupting.

Humid air can scatter back more of the light pollution than normal, but I suspect the bright skies you're seeing are more likely just due to the twilight which persists all night at this time of the year, even as far south as Austria.